Because Astral is a powerful school of magic. It has access to quite a few of the best spells in the game. Also the nature of the game permits you to not always place your Pretender in immediate danger (I.E. combat) so when you are faced with a situation where you are facing Pythium/R'leyh/Arco and to a lesser extent, the Astral 2 Mage nations that are gunning for your Pretender he can be regulated to a different duty.
The power of the Wyrm has always been early initial expansion, Astral helps that particular cause quite a bit, lengthening his already initial investment by a factor of 5~10 turns. In that timeframe you need to make him count for what he is doing. After that point if you are in conflict with one of the aforementioned nations, you have to switch the use of your Pretender, more than likely summoning/forging/gatewaying. Choosing a different path (Say Air or Earth) you would not be limited by that particular aspect, but still limited as you would in any game because as the game progresses, SC Pretenders become less of a factor as other nations can manufacture their own SC's that can potentially kill your Pretender without losing as much in the process.
These are of course advanced multiplayer strategies that you are the most vulnerable to. If you are trying to learn about the viability of using SC's by easiest example (like your Pretender) and the spell paths/uses of spells (as someone mentioned earlier) this is a valuable learning experience. Which is exactly why you started this AAR, correct?
You didn't start it to run up and into MP situations as supreme grognard, but with the knowledge of how an aspect of the game is used. But as a tool of learning and showing people your own mistakes/insights/observations/innovations.
Take it in that light and I'm sure you'll feel much better. What some of the more experienced players may not remember, or care about, is that a whole deluge of newbies are out there. Trying to learn. This is a tool for those as well as the players themselves. While people can sit here and spout advise all day, it's not going to aid you in playing the game until you figure out the entire aspects for yourself and reasonings. It would be pure folly to try to do so and you would lose alot of the magic that this game so eloquently provides.
Edit: One of the best counters to Communion/Teleporting/Covens is Fires from Afar and to a lesser extent Seeking Arrows, etc. Foul Vapors, Wrathful Skies, Lammashatas, Fliers all do very well at killing mid-large numbers of commanders.
Edit: I also suggested Earth and Air previously Arryn

But you have your own personal standards, didn't want Earth because you wern't familiar with it and Air, for whatever reason

. Astral is just the next down the line of simplicity to use.
[ February 10, 2004, 21:29: Message edited by: Zen ]